The text came through on the night of the first Welsh Labour leadership debate in Cardiff.
"Mark Drakeford just told Welsh Labour hustings in public: 'There is nothing courageous about calling for a People's Vote'."
The source, a keen supporter of another referendum, said pro-European Labour members were "furious behind the scenes".
But how many of Welsh Labour's roughly 25,000 party members are "furious" with Mark Drakeford's stance on a second referendum?
In a three-horse race where Vaughan Gething and Eluned Morgan unambiguously back the so-called People's Vote campaign, is Mark Drakeford's equivocation on the issue a trip-hazard in a contest seen by many as his to lose?
"We're pretty relaxed about it," a Drakeford supporter told me, while a senior Welsh Labour source said "he's dropping the ball on this issue but he's got plenty of balls to drop".
His rivals see it differently.
"It is the biggest issue of this campaign and Mark is on the wrong side of the debate," according to a member of Vaughan Gething's campaign team.
"This is definitely a weakness for Mark. The Labour Party membership is wholeheartedly behind the People's Vote."
That claim is based on an opinion poll released on the eve of Labour's conference in Liverpool back in September - 86% of the party's members indicated support for a public vote on the outcome of the Brexit negotiations.
It caused the party's top brass a headache during conference before they reached a compromise motion that was backed by the members.
Labour's policy: vote against a Brexit deal if it doesn't meet the party's six tests (spoiler alert: it won't), push for a general election and, failing that, as per the conference motion, "support all options remaining on the table, including campaigning for a public vote."
"I myself stand four-square behind the Keir Starmer set of proposals in this area," Mark Drakeford told an audience in Cardiff earlier this month.
He had just given a speech in which he outlined his analysis that austerity was a "key causal factor" behind the Leave vote and that his socialist agenda could help tackle the issues that drove some to back Brexit.
Although he regrets the outcome, it was a speech about shaping, not stopping Brexit.
Challenged by an audience member to wholeheartedly back another referendum, the Cardiff West AM explained his outlook: "If you come with me to knock doors in the Ely part of my constituency where people vote Labour but voted to leave the European Union, you will not detect, I believe, that change of heart that was there in some of the opinion polls.
"They voted that way then and they would vote that way again now and they are particularly likely to say that to you if they hear the message that I think I can begin to hear articulated, in the way that they hear it.
"We have to hear those arguments not through our ears, not in the way we think we are making them, we have to think about them in the way that they are heard by people who took a different view to ourselves.
"And when we say to them, if we are not careful with the language we use, 'you were lied to', what they think we are saying to them is, 'you were stupid, we understood because we heard the same lies and we came to a different conclusion but you heard the lies and you made the wrong decision - weren't you stupid?'
"Now, we don't mean to say that. The People's Vote never mean to say that but, I'm telling you, that is the risk that it is perceived in that way."
A member of the finance secretary's camp agreed, saying that although "the vast majority of Labour members want to remain in the EU they don't necessarily see it as the biggest issue in this contest".
It is not a view shared by Vaughan Gething's team.
"There hasn't been one party member that we've contacted that has said Brexit isn't a big issue," according to a team member.
The Cardiff South and Penarth AM has said "the time for fudge on this issue is at an end" and that Labour needs to lead the campaign for a second referendum - Plaid Cymru understandably says that Vaughan Gething and Eluned Morgan voting against their second referendum motion last month was a slice of the stickiest political fudge.
The Gething camp's Brexit strategy, according to another source, is to channel US TV drama The West Wing's "Let Bartlet be Bartlet" philosophy. "It's a case of 'let Vaughan be Vaughan' on this as he's naturally pro-EU."
It's a label that also sits easily with Eluned Morgan - a Member of the European Parliament (MEP) for 15 years.
The Mid and West Wales AM told me a few months ago that it would "make sense" to ask the public again whether they are "content" with their decision in 2016.
The People's Vote pledge is a cornerstone of her leadership pitch.
"Our problem is that Vaughan is saying exactly the same thing", a source from her campaign told me.
However, the Morgan team sees a potential weakness in Drakeford's "toeing of the Corbyn line" on Brexit and other issues.
"Mark is trying to behave like his disciple. I think that's going down really badly," they added.
A Gething source agreed: "He obviously just wants to carbon copy Labour in London.
"Sometimes it's just a 'who supports Jeremy Corbyn most' strategy.
"This contest needs to be about a specific policy platform for Wales."
Mark Drakeford is the man credited with crafting Rhodri Morgan's "clear red water" speech in 2002 to signal policy divergence between a Welsh Labour government in Cardiff Bay and Tony Blair's New Labour in Westminster.
A fully supportive Corbynista - he's even using the UK Labour leader's slogan of "For the Many, Not the Few" for his own campaign - Drakeford now wants to see that "clear red water" ebb away.
And, according to a Drakeford campaign supporter, for good reason.
"We've done lots of phone-canvassing and people want to know if the new Welsh Labour leader is going to support Corbyn.
"It seems to be a big issue, far more important than Mark's Brexit stance," they added.
Another agreed, pointing out that most party members who joined after 2015 did so to support Corbyn and are critical of Labour politicians attacking their own leader.
For many, the ubiquitous "Love Corbyn, Hate Brexit" slogan emblazoned on conference tote bags summed up Labour members' mood on Merseyside a few months ago.
In just a few weeks time, we'll see whether it's a slogan that tells us something about Labour members in Wales and their choice for leader.